1. Field of Invention
This invention relates to the field of liquid fertilizers prepared from wet-process phosphoric acid and especially to an improved method of treating weak wet-process phosphoric acid to significantly lower the level of magnesium impurities therein which would otherwise carry through to the liquid fertilizer and form magnesium containing compounds therein which would settle out during extended storage of the liquid fertilizer.
For economic reasons, phosphoric acid used in the production of fertilizers, including liquids, has generally been obtained from phosphate rock which is acidulated with sulfuric acid to precipitate calcium sulfate, the solids removed by filtration, and the weak acid then concentrated to a desired level by any one of a number of conventional evaporative techniques. Much effort has gone into development of processes which lower or minimize the effect on fertilizers of impurities which are present in the acid following concentration, with only marginal success being realized in most instances because of the high cost of impurity removal or inadequacy of the treatment procedure.
Particularly vexatious has been the problem of removing magnesium compounds from wet-process phosphoric acid during concentration of the acid because of the solubility of these compounds in the acid notwithstanding the fact that after production of liquid fertilizers from concentrated acid, there is a significant tendency for impurities such as magnesium compounds to form gelatinous sludges or precipitates at the bottom of storage facilities for the liquid product. Many reasons have been identified for the tendency of magnesium impurities to precipitate from liquid fertilizers during storage.
Most liquid fertilizers are prepared from wet-process phosphoric acid having an effective P.sub.2 O.sub.5 content of 68% or greater, and in order to obtain wet-process acid of this concentration, the weak acid must be subjected to evaporative processes which cause the resulting acid to have a relatively high final temperature. On cooling, the viscosity increases thereby making handling of the 68% acid more difficult. Also, the higher the level of impurities, the greater the viscosity of the acid. Compensation for these additive factors makes it desirable to ship the 68% acid in insulated tank cars which maintain the acid at a temperature level approaching that at which it was introduced into the tank car. In order to minimize problems of thickening of the acid during shipment, the acid during concentration should be subjected to processes which reduce the impurity content level. This makes the unloading and processing thereof much easier. The acid upon receipt should be in condition for ready transfer and reaction in a pipe reactor with ammonia to produce liquid fertilizer.
Liquid fertilizers during storage will form a gelatinous sludge if adequate steps have not previously been taken to remove deleterious impurities from the feed acid and, therefore, exhibit a limited shelf life. The length of the shelf life depends upon the ambient temperature encountered during storage and the impurities content that carried over from the wet-process acid. Of the impurities that may be present in the liquid fertilizer, magnesium is the most detrimental. Experience has shown that the acid used for making liquid fertilizers must have a MgO concentration of less than 0.4% (68% P.sub.2 O.sub.5 basis) to yield a product with accceptable shelf life when as little as 60% of the total phosphate is present as polyphosphates.
Although settling out of solids in the storage tank is a problem which causes concern to suppliers and retail outlets, the real problem is encountered by the user of the products when an attempt is made to apply the liquid fertilizer to the land using conventional spray nozzles and applicator tools. Solids present in the liquid quickly clog up delivery apertures causing costly downtime of the equipment and exasperation on the part of the user.
Rather than attempt to remove sufficient magnesium impurities after liquid fertilizer production and storage, the preferred procedure is to remove the undesirable impurities from the wet-process phosphoric acid during production thereof, thus avoiding sludge and solids accumulation in liquid fertilizer products at the time of application to the ground.
2. Prior Art
Efforts to remove deleterious impurities from wet-process phosphoric acid have taken many forms in the prior art and often involved addition of various agents to the acidic solution which were designed to effect precipitation of impurities for removal by filtration. In British Pat. No. 467,843 of June 24, 1937, it is proposed that hydrofluoric acit be added to the wet-process phosphoric acid and that the ratio of calcium and aluminum present in the mixture be controlled within a critical relationship. Carothers, et al. in British Pat. No. 1,024,924 of Apr. 6, 1966 teach adjustment of the mole ratios of aluminum to silicon in the acid and of aluminum to fluorine followed by addition of a soluble calcium orthophosphate in an amount said to be sufficient to react with the impurities and form a complex crystalline solid.
Crawford, et al. in U.S. Pat. No. 3,206,282 of Sept. 14, 1965 describe a process for treating phosphoric acid to remove undesirable impurities by concentrating the acid to a level of 75% by weight phosphoric acid or greater followed by addition of after-precipitate solids obtained from a previous mixture to clear concentrated phosphoric acid and then cooling the mixture to a level of 30.degree. to 120.degree. F. during a period of 20 to 36 hours. Filtration of the liquid is then said to remove substantial quantities of impurities.
Treitler, et al. in U.S. Pat. No. 3,379,501 suggest that impurities may be removed from wet-process phosphoric acid by adjusting the calcium ion level of the acid and then adding hydrofluoric acid in amounts to react with all of the cation impurities therein.
Carothers, et al. U.S. Pat. No. 3,442,609 of May 6, 1969 which is a CIP of the application which corresponds to British Pat. No. 1,024,924 suggest adding a sodium salt to the wet-process phosphoric acid in order to enhance the removal of impurities.
Legal, et al. in U.S. Pat. No. 3,481,700 of Dec. 2, 1969 describe a process for stabilizing wet-process phosphoric acid wherein the acid is dehydrated to a level of about 38% to 46% by weight P.sub.2 O.sub.5, cooled to a temperature of 20.degree. C. to 50.degree. C., agitated for a period from about 4 to 12 hours, allowed to stand for a period of 12 to 24 hours in a quiescent state and then subjected to filtration to remove solids.
Shearon, et al. in U.S. Pat. No. 3,528,771 of Sept. 15, 1970 provide a centrifuge operable to separate solids from sludge at a bottom of a settling tank installed between first and second stage evaporators. Settling is a primary means of removing solids from the main process stream. The sludge and entrained liquid from the settling tank goes to the centrifuge while the overflow therefrom is returned to the filtrate storage tank. Filtrate is fed to the first stage evaporator.
Moore, et al. in U.S. Pat. No. 3,642,439 of Feb. 15, 1972 suggest treatment of the phosphoric acid to limit the silicon content followed by maintaining the acid at a temperature of 50.degree. C. to 100.degree. C. for at least 15 hours to form a precipitate said to be a magnesium-aluminum-fluoride-phosphate complex compound which is then separated from the acid.
In U.S. Pat. No. 3,819,810 of June 25, 1974, Goldstein again suggests the addition of a compound containing fluorine to merchant grade acid having a P.sub.2 O.sub.5 assay of 52% to 54% to enhance recovery of impurities.
The Mills U.S. Pat. Nos. 4,136,199 of Jan. 23, 1979 and 4,243,643 of Jan. 6, 1981 suggest removal of metal ion impurities from wet-process phosphoric acid by addition to the acid of a precipitant comprising ions of calcium and fluorine to cause precipitation of a magnesium-containing precipitate. A preferred precipitant is one containing calcium fluoride, such as the sludge obtained by treating pond water from a phosphoric acid plant with a calcium-containing compound.
Williams, et al. in U.S. Pat. No. 4,248,843 of Feb. 3, 1981 propose treating wet-process phosphoric acid to remove impurities by heating the acid under conditions of temperature, pressure, time and concentration as to cause at least some of the impurities to precipitate as pyrophosphates of the relevant metal ion.
Parks, et al. in U.S. Pat. No. 4,299,804 of Nov. 10, 1981 again teach the addition of hydrofluoric acid along with alum to wet-process phosphoric acid, certainly for the purpose of increasing the precipitation of impurities for removal by filtration.